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Discussion papers
https://doi.org/10.5194/se-2019-69
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/se-2019-69
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 18 Apr 2019

Research article | 18 Apr 2019

Review status
This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Solid Earth (SE) and is expected to appear here in due course.

A revised map of volcanic units in the Oman ophiolite: insights into the architecture of an oceanic proto-arc volcanic sequence

Thomas M. Belgrano1, Larryn W. Diamond1, Yves Vogt1, Andrea R. Biedermann1,2, Samuel A. Gilgen1, and Khalid Al-Tobi3 Thomas M. Belgrano et al.
  • 1Institute ofGeological Sciences, University of Bern, Baltzerstrasse 3, 3012 Bern, Switzerland
  • 2Institute for Rock Magnetism, University of Minnesota, 100 Union St SE, 55455 Minneapolis, USA
  • 3National Earth Secrets Co., P.O. Box 1242, PC 130 Athaibah, Muscat, Sultanate of Oman

Abstract. Recent studies have revealed genetic similarities between Tethyan ophiolites and oceanic proto-arc sequences formed above nascent subduction zones. The Semail ophiolite (Oman–U.A.E.) in particular can be viewed as an analogue for this proto-arc crust. Though proto-arc magmatism and the mechanisms of subduction-initiation are of great interest, insight is difficult to gain from drilling and limited surface outcrops in submarine fore-arcs. In contrast, the Semail ophiolite, in which the 3–5 km thick upper-crustal succession is exposed in an oblique cross-section, presents an opportunity to assess the architecture and volumes of different volcanic rocks that form during the protoarc stage. To determine the distribution of the volcanic rocks and to aid exploration for the volcanogenic massive sulphide (VMS) deposits that they host, we have re-mapped the volcanic units of the Semail ophiolite by integrating new field observations, geochemical analyses and geophysical interpretations with pre-existing geological maps. By linking the major element compositions of the volcanic units to rock magnetic properties, we were able to use aeromagnetic data to infer the extension of each outcropping unit below sedimentary cover, resulting in in a new map showing 2100 km2 of upper-crustal bedrock.

Whereas earlier maps distinguished two main volcanostratigraphic units, we have distinguished four, recording the progression from early spreading-axis basalts (Geotimes) through to axial to off-axial depleted basalts (Lasail), to post-axial tholeiites (Tholeiitic Alley) and finally boninites (Boninitic Alley). Geotimes (Phase 1) axial dykes and lavas make up ~55 vol% of the Semail upper crust, whereas post-axial (Phase 2) lavas constitute the remaining ~ 45 vol % and ubiquitously cover the underlying axial crust. The Semail boninites occur as discontinuous accumulations up to 2 km thick at the top of the sequence and constitute ~ 15 vol % of the upper crust. The new map provides a basis for targeted exploration of the gold-bearing VMS deposits hosted by these boninites. The thickest boninite accumulations occur in the Fizh block, where magma ascent occurred along crustal-scale faults that are connected to shear zones in the underlying mantle rocks, which in turn are associated with economic chromitite deposits. Locating major boninite feeder zones may thus be an indirect means to explore for chromitites in the underlying mantle.

Thomas M. Belgrano et al.
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Thomas M. Belgrano et al.
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Geochemical compositions (XRF, LA-ICP-MS, EMPA) and bulk magnetic properties of volcanic rocks used for mapping in the northern Semail ophiolite (Oman) T. M. Belgrano, L. W. Diamond, Y. Vogt, A. R. Biedermann, S. A. Gilgen, and K. Al-Tobi https://doi.pangaea.de/10.1594/PANGAEA.899794

Thomas M. Belgrano et al.
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Short summary
We present a geological map of the different volcanic rocks present in the northeast Oman mountains. These volcanic rocks were erupted erupted at the seafloor, probably above a nascent subduction zone, and have since been transported by tectonic processes into their present, accessible position. The updated map allows us to understand the links between the different volcanic rock types and other geological features, including copper and chromite deposits, and will aid further study in the area.
We present a geological map of the different volcanic rocks present in the northeast Oman...
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